In the process control industry many process applications may produce unacceptable levels of aerodynamic noise. For example, modern power generating stations typically use steam turbines to generate power. The steam turbines require periodic maintenance and it is generally known to be more economical to continue steam generation during turbine maintenance than to completely shut down the plant. During turbine maintenance, a series of supplemental piping and valves, known as a turbine bypass system, circumvent the steam turbine and redirect the steam to a recovery circuit where the steam is repetitively recycled. It is understood that the process conditions within recovery circuit produce high temperatures and large pressure differentials (e.g., 1200° F. and 500 psid) that may create damaging vibration and high levels of noise within the system as steam is redirected from the turbine. To prevent these conditions from damaging the steam recovery circuit components, steam temperature and steam pressure must be reduced prior to entering the system.
Typically, to control the steam temperature and steam pressure prior to entering the recovery system, fluid pressure reductions devices, commonly referred to as diffusers or spargers, are used. Diffusers are aerodynamically restrictive devices that reduce fluid pressure and temperature by transferring and/or absorbing fluid energy contained in the bypass steam. Typical diffusers are constructed from a hollow housing including a series of passageways throughout the housing walls that connect multiple inlet ports along the interior walls to outlets along the exterior surface of the diffuser as described in U.S. Pat. No. 5,769,122 and U.S. Pat. No. 6,244,297 and are hereby expressly incorporated by reference. Generally, the passageways within these devices separate and divide the incoming fluid into progressively smaller fluid jets that subsequently reduce the pressure and the temperature of the incoming fluid.
Similarly, in control valve applications, valve trim, such as cages, may also encounter harsh conditions. For example, in Liquid Natural Gas (LNG) distribution applications, large compressors are used to pressurize the natural gas to liquid phase prior to introduction into a distribution pipeline. It is known that during compressor operation a potentially destructive condition known as “surge” may occur. The surge point of the compressor is generally defined as the operating point where the maximum pressure at minimum stable flow can be achieved for a given compressor speed.
Operation of the compressor at or below the surge point may cause unstable operation that may cause compressor surge to occur. For example, in normal operation as gas flow through the compressor system decreases, the fluid pressure increases to maintain flow, but near the surge point, the compressor can not impart enough momentum in the gas to continue gas flow through the compressor, causing gas flow to momentarily stop. As flow stops, the inlet pressure falls and the outlet pressure may become greater than the inlet pressure, which causes a flow reversal within the compressor (i.e., gas flow is momentarily from the outlet to the inlet). The flow reversal is maintained until an adequate pressure head develops at the turbine inlet to overcome the surge condition. If compressor operation continues near the surge point, the surge condition will repeat, causing repetitive flow reversals, until the process conditions change. The flow reversals associated with compressor surge create compressor thrust reversals that can cause unstable axial and radial vibration that can damage the compressor and create high levels of noise.
To avoid compressor surge from occurring and damaging the compressor, antisurge systems are built around the compressor. It is commonly known that antisurge systems require high capacity antisurge valves (i.e., large flow and high pressure valves). For example, antisurge valves may have 22 inch ports and operate at a 550 psi pressure differential. One of ordinary skill in the art can appreciate that these flow conditions create high mass flow rates that can produce very turbulent flow and create unacceptable levels of aerodynamic noise. To prevent unwanted noise and damaging vibration, antisurge valves also rely upon noise attenuating fluid pressure reduction devices.
Current fluid pressure reductions devices, such as the Whisperflo® trim, available from Fisher Controls International LLC of St. Louis, Mo., use multi-stage fluid pressure reduction designs formed from a stack of annular plates that define the multiple restrictive passageways between a hollow center and an outer perimeter. In such a device, the fluid moves through a series of passageways that create changes in radial and axial flow through a series of contraction-expansion fluid structures that substantially reduces fluid pressure by mixing the fluid flows and separating the fluid into numerous, distinct high velocity jets at the outlet of the device. These conventional devices are known by those of ordinary skill in the art to work best in applications with low to mid pressure-drop ratios; not in very high pressure-drop ratio applications.
High pressure-drop ratio applications may be identified as applications where the ratio of the pressure drop across the fluid pressure reduction device with respect to the inlet pressure exceeds a specified ratio, such as 0.93. Similarly, other applications are defined as high pressure-drop ratio applications when, relative to process conditions, the inlet-to-outlet area ratios of the fluid pressure reduction device facilitate sonic flow (i.e., fluid velocities greater than or equal to the speed of sound) at the final or outlet stage of the device. It is generally understood that sonic flow for compressible fluids in fluid pressure reduction devices means “choked flow”. One skilled in the art can appreciate that at choked flow, there is a discontinuity between upstream and downstream flow conditions. That is, with respect to the pressure reduction device and its internal fluid structures, the mass flow rate is exclusively proportional to the upstream pressure. It is typically these flow conditions (i.e., maximum mass flow) that produce sonic fluid velocities. When the fluid velocities approach the speed of sound, shock cells form within the fluid that contribute to unacceptably high levels of noise. In high pressure-drop ratio applications, conventional fluid pressure reduction devices quickly experience choked flow and are not acceptable in such applications where noise level and vibration are a concern.
To solve such a problem, conventional fluid pressure reduction devices typically reduce unacceptable noise levels induced in high pressure-drop ratio applications by placing a supplemental baffle around the fluid pressure reduction device to provide sufficient flow area at the periphery of the device to create a small, controlled pressure drop at the outlet stage. The controlled pressure drop induces a back pressure at the outlet stage to restrict outlet fluid velocities to subsonic flow. Unfortunately, large baffle areas are required for very high pressure-drop ratio applications and these types of baffle/attenuator devices cannot be easily placed in valve bodies for valve trim. This approach also significantly increases the manufacturing cost of large diffusers. Additionally, typical multi-stage fluid pressure reduction devices generally do not have enough physical structure within the housing to withstand the very high pressure drops and have been known to physically separate while under load, causing catastrophic damage to the valve body or the piping/duct system around the diffuser.
Other conventional approaches to improve traditional fluid pressure reduction performance in high pressure-drop ratio applications include decreasing inlet-to-outlet area ratios within the device, such as reducing the number of inlets available within the device. Unfortunately this technique reduces overall fluid capacity of a system or valve. To maintain a given fluid capacity for a device with such a decreased inlet-to-outlet area ratio, the overall stack height of the fluid pressure reduction device must increase. This technique is not viable in valve trim or diffusers as increases in stack height may make the structure too large to fit within valve bodies or duct work and may be too costly to manufacture. Accordingly, it is desirable to create an improved fluid pressure reduction device for high pressure-drop ratio applications that may be suitable for diffusers and/or valve trim.